1. Field of the Invention
The present invention relates to ultrasonic motors.
2. Description of the Related Art
Japanese Patent Application Laid-open No. H9-117168, for example, discloses an ultrasonic motor that generates elliptical vibrations by combining longitudinal vibrations and torsional vibrations, and rotation drives a rotor. FIG. 1 of Japanese Patent Application Laid-open No. H9-117168 depicts an exploded perspective view of a vibrator. The vibrator has a structure in which a plurality of piezoelectric elements is arranged between elastic bodies that are cut obliquely with respect to an axis of the vibrator. Positive electrodes of the piezoelectric elements are divided into two groups. These groups will be called as Phase A and Phase B electrodes.
The longitudinal vibrations can be generated in the bar-shaped vibrator by applying alternating voltages of the same phase to both Phase A and Phase B electrodes. On the other hand, the torsional vibrations can be generated in the bar-shaped vibrator by applying alternating voltages of opposite phases to both Phase A and Phase B electrodes. A position of a groove in the vibrator is adjusted such that a resonance frequency of the longitudinal vibrations and a resonance frequency of the torsional vibrations substantially match. When alternating voltages that differ by n/2 phase are applied to Phase A and Phase B electrodes, the longitudinal vibrations and the torsional vibrations are generated simultaneously, thereby generating elliptical vibrations on a top surface of a bar-shaped elastic body. In this state, by pressing the rotor on the top surface of the bar-shaped elastic body, the rotor can be rotation driven in a clockwise direction (CW direction) or a counterclockwise direction (CCW direction).
The ultrasonic motor disclosed in Japanese Patent Application Laid-open No. H9-117168 has various drawbacks. For example, as shown in FIG. 1, both the piezoelectric element and the elastic body are necessary, the elastic body must be cut obliquely, and the groove must be made in a portion of the elastic body to match resonance frequencies of the longitudinal vibrations and the torsional vibrations. Thus, the overall structure of the conventional vibrator is very complicated.